Sahih Bukhari | Volume 1 | Hadith Number 317

Narrated by 'Aisha
Fatima bint Abi Hubaish used to have bleeding in between the periods, so she asked the Prophet about it. He replied, "The bleeding is from a blood vessel and not the menses. So give up the prayers when the (real) menses begin and when it has finished, take a bath and start praying."